Electrolytic titrimeter



Dec. 16, 1952 E. 1.. ECKFELDT 2,621,671

ELECTROLYTIC TITRIMETER Filed Nov. 21, 1944 2 SHEETS-SHEET 1 Allll 1/. ""I'" INVENTOR EDGAR LECKFELDT BY 4 flaw A TTORNE Y Dec. 16, 1952 E. L. ECKFELDT 2,621,671

ELECTROLYTIC TITRIMETER Filed Nov. 21, 1944 2 SHEETS-SHEET 2 nth]; 121;

EDGAR L ECKFELDT Patented Dec. 16, 1952 ELECTROLYTIC 'II'I RIIVIETER'- Edgar-L. Eckfeldt, Ambler, Pa., .assignorrto -lgeeds and Northrup Company,.Philadel'phia', Pa a" corporation of Pennsylvania Application November 21, 1944, Serial No.-564;536v

13 Claims.

This 'iinvention' relatestoimethods" and apparatusior determining'orestablishing compositional chara'cteris'tics'of substances, and'has'for an object the provision'of a reliable systemby means of "which a selected compositional characteristic may be readily ascertained.

In many processes it is important accurately to measure compositional characteristics of substances: These measurements may be made on,

for" the control of, reagents used in the. treat ment ofsubst'ances as well as forvolumetric' de-- termination of the character of the endproductiv ofa reaction system; Heretofore, the measure ment'of'a compositional characteristic-of a'substance; as" accomplished by volumetric analysis;

involved the addition of area-gent of known concentration in known quantity" sufiicient to just produce'a stoichiometriareaction', which commonly is referred to as bringing the solution to an end-point.

Moreparticularly, in aeidimetry and alkalinietry, areagent, either a base or antacid, as'irequired, of" known concentration isadded'to the lin'own volume of a'sa-mpleuntil a desired end point is attained;

and the volume of the added base or acidity oralkalinity of'the original'sample may beas'certainedl Systems of "tliis'character require notonly highly accurate'measurement of the volume of theadded reagent; but they also require e'qually' a'ccurate knowledge 1 of the concentration of "therea'genti Thepreparation of reliable standard solutions invo'lve'sthe expenditure of considerable efi'ort and frequently indirect, tedious, and time-com sumingjtechniques must'be employed. Moreover, the elforts so expended may be nullified'by-the inevitablechanges in such solutionswhich often take-placewith the passage of time. This is es-' pecially true with dilute solutions where the slightest amount of *impurity might cause change which would'be very material. Apparatus which employed in volumetric analysis mustbe handled with skill, and errors may arise in measurement's'of volumes of standardized solutions when inadequate precautions are observed; Furthermore, the effect of'temperature' onthewolumes of solutions is appreciable and in careful work introduces an additional factor for which allowance must be made. Since both the concentration and the volume of the standard solution arerequired in calculating or determining arrunknown, in volumetric analysis an unceranswer. error in either of these will afiect the final-result;

Knowing the concentration acid, the

It-lis,-'.therefore; flan-: objector;thespresentlinvene I tion not onlyeto "eliminate someot the'many steps; required.- in carrying. out-.- conventional i titration,.. but also .to: eliminate the constant needionstand';

ardized reagents as a part of titrimetricsystems. It is a further object of the .inventionrtolutillize an. electrical methodand-system. for producing a: known current which is so 1 utilized as.

to replace the numerous standard-solutions here-- tofore required in titrimetric systems.

Itis'a further object- 0ft the invention toproduce electrically a reagent, which related in.

amount to the coulombsof! electricity, which: is-

utilized in place of standardized solutions and the burette equipment associated therewith.

In carryingout the invention, l an electric current is applied electrolytically to change :thecomspositional characteristic of substances in manner such that the desired measurementmay be attainedintemnsfiof an electrical quantity which iscOrrelated'With the aforesaid change irnth'ev compositional characteristic. For example, in determining: the total acidity or the total alkalinity of solutions, an electrical current is. soapplied to the solution as to electrolytically,produce a neutralizing-or titrating-reagentl In accordance with the present inventiornadvantage? is" taken. of the" factthat the" rate of productionaof: the neutralizing. reagent :is: proportional: to;. or may be readily: correlated with, the applied: electrical current. Accordingly; the?- in vention: makes possible; continuous operation; of. systems for determiningv compositional characteristicstinterms; of thesrate of flow of the. unknown materialand' inzterms "of theeapplied .elec-- trical current. Sincerthere is correlation-between the rate ofv productionof the. reagent and: the current flowing, the quantity of reagenti for-med in a given time may beknown: in terms. ofco-ulombsz For batchoperation, the-amount of reagent formedis directly determined-byascentainingjthe coulombs of electricity.

Further-in accordance withtheinvention, suitable indicatorsor indicating systems, wellknown to those skilled inzthe art, may-'b'e-utilizedfor the determination of the arrival of-a'n unknown-substance at a given end-point;

From the foregoing. it will be seenthat the presentinvention contemplates controlling by titrating agent. Thus, one aspect of the invention contemplates the electrolytic generation in situ of the titrating agent and the control of the amount generated by controlling the current employed in the electrolysis in response, as for example, potentiometrically, to the unreacted amount of one of the reactants, and the measurement of the current used to generate the titratin agent. Where the concentration of a constituent in a solution is determined potentiometrically, or where there is potentiometric sensing of the unreacted amount of one of the reactants, it is desirable that the circuit externally of the cell or reaction zone in which the electrolytic current is varied in response to variations in the sensed potential be separate or electrically isolated from the potentiometric detecting circuit.

The term end-point as used in the specification and claims means that a given compositional characteristic has a known value, whether that value he a particular magnitude of the compositional characteristic or whether it be an absence of the particular substance originally in the material under test. Hence, the end-point serves significantly to indicate when to terminate the electrotitrating action.

For a more complete understanding of the invention and for further objects and advantages thereof, reference is to be had to the accompanying drawings, in which:

Fig. l diagrammatically discloses a continuous sampling system, which may also be used for batch operation, embodying the invention;

Fig. 2 illustrates a preferred modification applicable to Fig. 1;

Fig. 3 illustrates a system of control applicable to the system of Fig. 1;

Fig. 4 illustrates a modification applicable to the system of Fig. 1; V

Fig. 5 illustrates apparatus for, and a system of, producing electrotitration capable of being reversed;

Fig. 6 illustrates a further modification, in accordance with the invention; and

Figs. '7 and 8 illustrate, partly in section, ap-

paratus suitable for carrying out the invention as shown in Fig. 1.

Referring to Fig. l, the invention in one form has been illustrated a applied to the determination of a compositional characteristic of a fluid flowing through a pipe IEI. This fluid may be a liquid used in an industrial process and with respect to which a particular compositional characteristic is of importance. By means well understood in the art, a sample may be continuously withdrawn from the pipe ID by means of a small pipe H, which delivers the sample into a constant-head receptacle l2. A constant head is maintained within the receptacle I 2 by reason of an overflow pipe I 3 which may be adjusted within the receptacle H, a pointer 14 being arranged to indicate on a scale 5 a particular setting of the inlet opening of the overflow pipe [3. The sample of the liquid, under the control of a valve 16, is discharged by pipe l'a into a flow channel l8, having a suitable exit pipe IQ for discharge of the liquid therefrom.

Disposed within the flow channel l8 are means for producing electrolysis comprising electrodes 29 and 2! supplied with current from a suitable source of supply as indicated by the battery 22. The magnitude of the current is under the control of a variable resistor 23 and by means of an ammeter or other indicating device '24, indication is had of the current flowing in the circuit. A switch 25 is provided for opening and closin the circuit.

Though other forms of indicators may be utilized, as will be explained hereinafter, in Fig. 1 the end-point or other value of the compositional characteristic is ascertained by a system for indicating changes in the potential be tween a pair of electrodes 26 and 2? connected to a suitable indicating device, such as a galvanometer 28 and to a potentiometer resistor supplied from a battery SI. The resistor 36 preferably has a scale 32 associated therewith so that the contact 33 may be set for the production of a predetermined potential in opposition to that developed between the electrodes 26 and 2?. Normally the setting of contact 33 is such that the pointer of the galvanometer 28 is at its neutral or null position when the solution surrounding electrodes 26 and 21 is at the desired endpoint.

The present invention is characterized by the utilization of a known electrical quantity which directly determines the change required in said compositional characteristic to bring it to the desired end-point. For example, if the particular fluid flowing through the pipe it comprises an aqueous solution of hydrochloric acid, the contact 33 is moved to such a position with respect to the scale 32 that the pointer of the galvanometer 28 will indicate zero when the endpoint is attained. This material flowing through the sampling pipe I l and into the constant-head device 12, flows into the channel It and is there subjected to electrolysis. The electrolytic action between the anode 23 preferably of silver, and the cathode 2!, preferably of platinum, produces chemical reactions characterized by the union of the chlorine from the hydrochloric acid with silver from the anode to form silver chloride. Silver chloride is practically insoluble, and. hence, is removed from solution. At the same time, hydrogen is given off at the cathode 2|. The rapidity with which the silver chloride and hydrogen are formed depends upon the magnitude of the current flowing between the electrodes 29 and 2!.

The resistor 23 is, therefore, adjusted to in crease the current flow until the pointer of the galvanometer 28 reaches its zero or null position, thus indicating that the end-point of the solution of hydrochloric acid after electrolysis, has been reached; in other Words, indicating that all of the chloride ions have combined with the silver, leaving a solution free of hydrochloric acid. The indicating system may be responsive to change in the pH or to the silver ion concentration, depending upon the nature or kind of detecting electrodes used. In either case, a potential change is produced in the region of the end-point or the equivalent point. For utilization of pH values, any suitable detecting electrode may be utilized, such. as the glass electrode, the hydrogen electrode, or the antimony electrode in combination with a suitable reference electrode such as the calomel electrode with a potassium chloride bridge.

So long as the concentration of the hydrochloric acid in the solution passing through the pipe ii] remains constant, the relationship established between the flow of the solution through the pipe ll and the how of current, as indicated by the meter 2 3, between the electrodes 29 and 2| remains constant. Therefore, it will be apparent that if the concentration of hydrochloric acid in the solution passing through the pipe It should increase; it: is: possible todecrease=-the headwithinthe containerl 2"-'-bylbwering thepip'e of the fluid into the channel I8 is decrea-se'dfby" an'amountwhich may *be-determined-by reading the-scale" I5 This decreases thewate oi "fliavv waf the fluid intothechannel I 8 and the-rate-of flbw of the solution past the electrodes 20' and 21: In consequence, the"material 'will remain"between the' electrodes 20 and 2"! 3 for- 'a' greater-"length of'=' Theprovision Y of the additional ti-i'ne permit's the complete removal' of the hydrochloridacid from thesolution: of" course, the"same= result may be accomplished by leaving the ffuid 'fl'owcon"- stant,'. at its original value; and increasing the flow-of current; Thedependentrelationshiobe tvleen the-two rates of fl'ow being" l'nown, thehydrochloric" acid concentration of the original stream passing= through -thepipe -l 0' may "be read ily ascertained;

Uponconsideration of the foregoing; it will b'e-seen'that as theconcentrationof-"hydrochloric" i acid' in the fluid passing through the' pi'pe I 0" increases', the current required to produce neutralization" or removal of the hydrochloric acid" from a fixed rate" of" flow of"samplernust" also be in creased; Also; that with'a' givenflow ofcurrent; the rate of" flow'of" the' fluid. within the channel l8" must be decreased as the" concentration of hydrochloric acid is increased.

Theconductivity of'the hydrochloricacidsolution shouldbeadequate: If the: conductivity is low, it is desirable; ini accordance with the in. vention";v to add to the sampling, stream asubstance which, thoughlneutralin other respects, does increase the conductivity. of. thesclutiom Such a substance for. example;sodium.nitrate, may be conveniently addedfi'oma suitableisource of supply (not shown) from which there extends an" inlet" pipe. 34 which leads. to. a. constantlhead container 35" and'flows under .the' control of :valve 38; by "pipe'31', to thefiow channelllBI Theefict of the: added sodium nitrate. is to. reduce the re: sistance of the solution; that-is, .fora given zvoltage; a greaterflow of f current Ana-yv be produced. between the electrodesi-d andJZ L. Withorwith- 0 out; the added.v sodium nitrate,- the: electrolysis or: electrolytic action is -.dependent upon theL-maglnifiidev of? theu currente flowing: between 1 electrodes 2S0 andlZll Consequently, a .measurement--of-.the-

magnitude of lthe. current .flow provides anlacc'ue rate.determinationctthe rate at-whioh the: silver chloride. is -formed. When .it is formed. at a rate which. removes .the hydrochloric: acid. froml'the' solution, the value of the original concentration thereof may be readily. determinedinterms-" 0f current flow.

Itwillnowbe readily understoodfthatall ofj'the hydrochloric acid maybe-removed irom these lution: but an end-point short: of" complete res moval of. I the acid -may be preferred; as. for ex ample, when automatic operation of thesystemis desired. If suflicient hydrochloric. acid." is re. moved so as togbringthez solutiontto a predeter mined or .readily. ascertained" concentratiori, the original concentration thereof may berea'dily determinedbysuitable calculations. in'somezapplicationso-f' the inventionit may be desiredto have. theend point short ofthe complete removal of the-substance being meas= ured, as: for? example the hydrochloric acid; in.

whiclicase-agaim ai suita'b'l c-aicumtion" will yiel'd" the original concentration.

Eet the rouowmg: symbols be: definedi r current" between the' electrodes 20 and? 21. which is 'required 5 to 1 cause the reaction with the substance being titrated;

R rate of flow' of solution containing the sub stancebeing titrated which passes i'nt'o (than: nel' -IBZ' N=normali'ty of? the 1 solution with respectto the substance' being titrated;

B the number of chemical equivalentsL- t ams;-

The well-known relationsnip between the quantitycf current which: passes, and: chemical equivalents given bythefollowing' equation zand-substituting'the value of: from. (1:) into the Equation 2, the following equation isrob tained:

i c 96,500V

If? Land :t are: amperesvand' seconds lrespectivei l'y, andiltisconstantxover the timet; Equationlii may; be" written:

- It r: A 96,5OOV' vWh'entthei'rate of. flowliisiconstant; theya'riable Vi may; bexsubstituted: by its: equivalent t Rt. andwhenr'R is'elitersperi second:

This last Equation 5 shows" the interrelationship between" the normality of the" solution; the current, and therat'eof flow of the-solution; The conversion .ofnormality to other expressions of concentration and the calculation, from the nori mality, of the weight of a substance'-in"'a"piarticui larf'volume arewell k'nown operations to those skilled :iii. thetart.

From the*foregoingiequa-tions"itWill?beunder tlie ilow channel 'I 8; theammeter2 l may"be=ca1i brated directly interms of the compositiona'l characteristic under'measurement. Hence; the meter-T4- may'also take the form of a" recording ammeter and the chart thereof 'may'be 'calibrated directly in terms of the compositional char'ac tenses:-

Since the rate of produc tion of the titrating agent is known in terms of current 'fio'w; the sys= temoffiFig; 2' is preferably utilized in connection with the flow'ch'annel arrangement" of Fig? 1E Specifically; the detecting device "or galvanometer' 28a is arranged to control,.as by means-Ora sin'gleipole double-throw "switch 3 8 thef energizati'on from supply lines-SI and"S2 and thedirection of rotation of a reversible motor39' arranged to'ad'j ust the setting-of the contact23a of resistor 23. B'y-asimple controlof'this" character, thecurrent"- flow is maintained at' such a' value as to maintain the sample at its predeterminedentl point. Hence; the continuousrecord made-1 by a? recordingiammeter 2411' shows tlicvariatiotr of the compositional characteristic of the sample with respect to time. With Fig. 2 utilized in the system of Fig. l as above explained, there is provided a current regulator having a sensing circuit including the pair of voltage-producing sensing electrodes 26 and 21 neither of which carries an of the direct current flowing between generating electrodes 20 and 2|. Also included in the sensing circuit is a means such as potentiometer resistor 30 for developing in that circuit a potential difference representative of the selected end-point of the constituent in the electrolyte in vessel or flow channel I8. The current regulator also includes a power circuit including generating electrodes 20 and 2| in the electrolyte.

The sensing circuit externally of said electrolyte is electrically isolated from said power circuit. The ammeter 242) comprises a current mewuring means in the power circuit which circuit also includes suitable means, as resistor or rheostat 23 for establishing the magnitude of current flow between generating electrodes 20 and 2|. The magnitude of the direct current will depend upon the setting of contact 32a relative to resistor 23. The motor 39 forming a part of the current regulator adjusts contact 32a as long as there isor for the duration of-a difference between the potential developed in the sensing circuit by potentiometer resistor 30 and the voltage developed by sensing electrodes 26 and 27. As the concentration of the solution is brought to an end-point, the end-point potential of the potentiometer balances the voltage from the sensing electrodes 26 and 21 and the motor 39 comes to rest. Thus the system brings the concentration of the constituent in the electrolyte to an end-point and maintains it at that end-point.

If it is desired to maintain at a predetermined value the compositional characteristic of the material flowing through the pipe It, the control system of Fig. 3 may be combined with the system of Fig. 1. In this case the galvanometer 28a again serves, through a single-pole double-throw switch 38a, to control the direction of rotation of a motor 39a to regulate a valve 40 to vary the rate of flow into the pipe in of a reagent supplied from a suitable source (not shown) so as to maintain at a predetermined value the compositional characteristic.

In Figs. 2 and 3 the motors 39 and 39a are shown with forward and reverse windings or coils F and R which are selectively energized. They are energized for the time interval during which the galvanometer 28a maintains the energizing circuit closed. the operation of the motor, 39 or 390., produces an adjustment or regulatory action which returns the galvanometer 28a to its neutral or zero position.

Other systems with additional refinements may be utilized in conjunction with, or in place of, those illustrated in Figs. 2 and 3. For additional disclosures of suitable systems reference may be had to United States Letters Patent No. 1,530,833, Fig. '7, to Keeler, and No. 1,918,021, Fig. 6, to Doyle.

If the requirements of the particular processes are such that continuous measurements are unnecessary, the system of Fig. 1 lends itself to batch operations. For example, whenever a compositional characteristic is to be determined it is only necessary to remove all material from the flow channel it and then by opening the valve IE to fill it with a fresh sample derived from the main stream flowing through the pipe H). The

In each system, of Figs. 2 and 3,

overflow or outlet pipe I!) will automatically drain off any excess material and leave in the fiow channel a known quantity of the fluid under measurement.

For a batch operation, it will again be assumed that the fluid under measurement is a solution of hydrochloric acid and that the position of the contact 33 corresponds to the desired end-point. In this case, the electrolyzing current is preferably kept constant by suitable adjustment of the resistor 23. By maintaining the electrolyzing current constant and measuring the time of aplication to the solution in the channel |8 required for the attainment of the end-point, there is readily determined the amount of reagent produced or the extent of the chemical reaction which removes from the solution all of the hydrochloric acid. Instead of maintaining the magnitude of the current constant, the meter 24 may be an ampere-hour meter which registers the time integral of the current. The amperehours or the coulombs of electricity required to bring the solution in the flow channel Hi to the end-point may then be utilized to provide an indication of the original concentration of the hydrochloric acid in the solution. As a matter of fact, such an ampere-hour meter may be calibrated directly in terms of acid concentration. Where the current is not maintained constant, the system of Figs. 1 and 2 (already described) will be utilized. Of course, as the end-point is attained, the switch 25 is opened to terminate the electrolysis.

In electrotitration systems constructed in accordance with the present invention, it will be observed that there have been eliminated the use of all standard solutions and the need for careful measurement of volumes of such standard solutions. The entire titration system of the illustrative examples is accomplished in terms of current flow or in terms of coulombs.

As will be later explained in detail, the electrodes 20 and 2| for continuous measurements are located up-stream with respect to the detectmg electrodes 26 and 2? to assure mixture with the sample'of all of the electrolytically produced titrating agent before measurement. The detect- 1ng electrodes 26 and 21 are accordingly so located as not to be affected by current flowing between electrodes 20 and 2|. With such disposition of the electrodes, and with electrodes 20 and 2| in a power circuit insulated or isolated from the detector circuit including electrodes 26 and 2! in all parts thereof external to the electrolyte in the flow channel N3, the current between electrodes 20 and 2| does not aiTect the potential developed between electrodes 26 and 21. That potential 1s developed solely as a function of constituent concentration.

As another example of determination of the concentration of acid, it will be assumed that a solution of sulphuric acid is passing through the pipe H] and that the sample thereof is being discharged at a constant rate from the pipe into the flow channel l8. If a current is caused to flow between the electrodes 29 and 2 l, respectively of silver and platinum, there will be a migration to the cathode 2| or" hydrogen ions and silver ions. The migration of the silver ions will interfere with the cathodic formation of hydrogen. In accordance with the preferred form of the invention, there will be preferably added, by means of the constant-head vessel 35, a solution of sodium chloride fed thereto by way of pipe 34. The constant-head device is not in this case utilized to produce ;-a constant how .but ,to assure the flow ofanexcessiof sodium chloride overand above that necessary-to the chemical reactions now tobe described. Neither the concentration of the'sodium chloride solution nor the amount fed into the flow channel'iS is important tothis aspect of the invention. Phe principal requirement is that it be-in excess of theamount required for thechemical reactions. Consequently no measurement of the addition agent, the sodium chloride, is required.

With the mixture ,ofthe sulphuric acid solution and the sodium chloride solution f'fiowing through thechannel LlSthere willbefproduced, under the'infiuence of the current jflow between the anode 29 and the cathode 25.,"reactions which include the'followin'g: Hydrogen ions-"at the oathode form hydrogen; 'atthe-anode, silver ions are formed which "subsequentlyireact withthe chloride ionsifrom the addition-'agent to form silver chloride which is insoluble and, hence, is removed-from-solutionand does-not interfere with thedischarge of the hydrogen ions at the cathode. Otherwise, silver -sulphate would-form and would be diffused-through the solution. In-this manner; the-cathode reaction is-controlledso that the removal of the acid-imparting materialis directly related to the electrolysis with-*aminimum, orabsence of, interference from other -reagents present.

The normality of the sulphuric acid solution is'given by the same Equation -5-aswas-previously-used for expressing the'normality'of thehy- 'drochloric'acid solution. Let it be=-supposed that the current as registered by-the-meter 2 4 is 0.162 ampere, and that the rate of flow o'f'solution into channel is 550.0402 cubic centimeter per second. The normality ofthe solution as given by Equation5-is:

The use of addition agentsds-not limited' to the illustrative example. "If the compositional characteristic is not of;itseli affected in a desiredway by-e1ectrolysis, the 'addition-agent may' be suchas to produce "under electrolysis 'a "further reagent which will react therewith and which -may be utilized forthe determination of the compositional characteristic.

'Itwill now Joe assumed that the-percentage or concentration of sodium sulphi-te in a solution passing through "the pipe Hi'isto be determined. A continuous sample is withdrawn and discharged-bythe pipe H into the fiow channel 1-8. Inorder quantitatively to determine thee-mount of sodium sulphite -present, an addition-agent is added "by means of-the constant hea'd receptacle 9 5. "For example, a suitable iodide or bromide may be usedastheaddition-agent. =Specifically, it-will be assumed that sodiu-m iodide is supplied by the pipe 31 in quantity in-excess'o'fthat which will be utilized in 'the subsequentreactions. In accordance with the invention, current is-supplied to the electrodes 2 3 a and 2- I which electrodes "in-this casemay both'be of an-inert material such,

for example, as platinum. The electrolytic ac- 'tion produces reactions, the most significant of which includes the-production of hy'drogen at the cathode and at the anode conversion of iodide ions to iodine. The iodine is an oxidizing agent which acts upon the-sulphite=to-convert.it .tdsulphate, with .a concurrent formation of iodide.

The formation of the iodine, which voxidizes the sulphite to the sulphate, is produced at i'airate 10 determined by the .current flow. By utilizing a suitable indicator which will give a definiteresponse when all-of the sulphite has been con-1 vertecleto sulphate, the concentration thereof in the original solutionmay be readily ascertained by the current required to produce the necessary amount of the oxidizing agent, the iodine. In this case, the appearance inthesolution of an excess of iodine indicates the desired end-point; Either the materialbeing titrated or the electrolytically produced ti-trating. agent may. be used .to indicatev end-points," depending upon the-particularreactions which occur.

The indicating system which has already been described and comprising the electrodes L26 and 21 -maybe used for determination of vthe end point ;.provided-one of the electrodes, the electrode "Z'tghe of platinum or other suitable ma terial immersed in the solution .and functioning asan iodine electrode, in manner well understoo'dlby thoses'killed'in'the art. As befoirathe contact T33 win he preset with reference .to the scale32 soithat'when the end-point is attained the .fgalvanomete'r fpointer will be at its neutral .or null position.

'It Will be readily understood that the foregoingqua'ntitative determination of the sulphite may alsdbelob'tainedjby a batch process'in which the meter24 will'measure the coulombs of electricity required to bring the batch or "fixedqu'a'ntityof the sulphit'e solution to its end-point. 'In' this case,'.the 'switch25 will be opened as the "endpoint is reached.

"When'a coulom'eter or other suitable means "for measuring coulombs is employed, the resistor 23 maybe fixed and the current may vary in manner as may be determined by the other parameters of the. system includingthe-resistance off the electrolyte.

T'Inisome caseslaterto be described,'apparatus is employed which permits a virtual, or real, separation of .thecathodic and anodicregions except forthemigrationof ions produced "by the flow of current. This may be readily done "with referenoe't'o the flow channel I8 by substituting the arrangement of Fig. '4 for that 'disclosed'in the rectangle of Fig. 1. With this substitution, the fiow .cha-n'nel 1'8 will bejprovided with aysepara'te compartmentil. Byway of example, vit will be assumed the anode 2B is located in compartment 5.l for. separating the reaction producits formed at the anode 20 from the solution in the flow :channel 18. .Any suitable .materialmay comprise .a separating diaphragm, such as parchment paper, or unglazed ceramic material 5.2.

In .every application of the invention, a'nI-lec- .trolyte is acted upon by the electrodes 12'!) and 2| that is to say, under the action of an electricpotential there will be a migration .ormoxlement of gpositively charged ions to .thecathode and a migration or movement of negatively charged ions to't'he anode. There will.be migra- "tion :of ,the hydrogen ions to the cathode and migration of chloride ions to the anode. The hydrogen ions uponv reaching the "cathode 2 twill be converted into hydrogen, which is ,given .off at the .cathode. Concurrently the :chloride .ions uponlarrivalattheanode 20 unite with'silver'ions "formed "as a-;result -.of the :electrolytic .action. Quantitatively. these greactions depend -.upon \the m gnitudetofzcurrentwflowing :between electrodes and Z1... Y

- the icontinuous zprocess, when the current isiof sufiicienkmagnitude to convert at thecathode all of the hydrogen ions which impart to the solution in the flow channel [8 the acid characteristic, the end-point will have been attained. This removal of the acid-imparting material is relatively independent of the reactions taking place at the anode. Hence, the anode may be separated from the cathode 2| except for the effects directly produced by the flow of current therebetween and thermal difiusion. The latter may be reduced to a very small magnitude by proper construction of the system.

In some cases there may be simultaneously produced an acid at the anode and a base at the cathode. If acidity or alkalinity were to be determined, the simultaneous and opposite reactions might nullify each other. By physically isolating the products produced at the anode and the cathode, electrotitration for a base or an acid, or both, may be carried out.

Another simple form of apparatus in which the two electrodes 20 and 2i are isolated has been illustrated in Fig. 5. The electrodes 20 and 2| are immersed in the two legs 54 and 55 of an H-tube 56, which are separated from each other by suitable material such as parchment paper or unglazed ceramic material 52. As further shown in Fig. 5, the two electrodes 20 and 2| are connected through a reversing switch 51 to a series-circuit comprising the meter or coulometer 24a, the resistor 23, and the source of current, the battery 22. In accordance with the invention, a system of this type lends itself to the determination of basicity of a solution of sodium hydroxide. A known quantity of sodium hydroxide solutionin mixture with an excess of sodium sulphate is introduced into the leg 55 of the H- tube 55 while a neutral solution, such as sodium sulphate, is introduced into the leg 54. The reversing switch 5'! is then closed in a direction such that current flows from the anode to the cathode 2|. There is also added to the solution in the leg 55 a suitable color-indicator of a pH end-point. This indicator may be phenolphthalein or methyl orange. The electrolysis resulting from the passage of current produces anodic reactions which discharge oxygen, thus removing hydroxyl ions from the solution. When the resultant change in the pH value of the solution reaches the critical end-point for the indicator used, there will be a significant change of color. Phenolphthalein is colored in basic solution. When the pH of the solution reaches a predetermined value there is a characteristic color change to colorless produced by the phenolphthalein. This characteristic change in color indicates the end-point for the phenolphthalein in"- dicator. If methyl orange is used as the indicator, a characteristic color change (from a pink in an acid solution to a characteristic yellow in a basic solution) occurs at a difierent but characteristic pH value of the solution. In either case, the compositional characteristic, the pH value, is known by the color change which takes place. Hence, there may be determined the basicity of the sodium hydroxide solution which was added to the leg 55 of the l-l-tube 56.

As a numerical example, the extent of removal of hydroxyl ions is given by the Equation 3 previously set forth and the symbols again bear the significance previously given them. The meter 24a gives the quantity Q for the titration and this, with the measured volume ofsolution employed, suffices to give the normality of the sodium hydroxide in the solution. Let it be assumed, for example, that a volume of 0.0250 liter 12 of solution containing sodium hydroxide is con tained in the leg 55 of the H-tube, and that the quantity of current as indicated by the meter 24a is 97.3 coulombs. The normality of the original solution with respect to sodium hydroxide is given as follows:

In accordance with the present invention, advantage may be taken of the character of the chemical reactions which take place at the anode 20 and the cathode 2! if these reactions reverse, upon reversal of current fiow. If it is suspected that the electrotitration has proceeded beyond the end-point for a given indicator, the reversing switch 57 may be operated to reverse the direction of current flow between the electrodes 20 and 2|. Such a reversal of current fiow reverses the functions of the electrodes. The electrode 20 then becomes the cathode and under the electrolytic action, hydrogen ions present .within the solution are destroyed at the cathode 20 with production of hydrogen. This reaction, by eliminating the hydrogen ions, increases the relative basicity of the solution. The reversal of the electrolytic action may be continued until the characteristic color change is attained, at which point the reversing switch 57 may be moved to its open position. If the meter 24a has been utilized to measure the coulombs of electricity rerequired on the first operation, as well as the coulombs to electricity utilized in the reverse operation, the latter is subtracted from the former to obtain the coulombs which were necessary to bring the solution to its predetermined end-point.

From the foregoing example it will be readily understood that a solution of sulphuric acid in mixture with sodium sulphate may be placed in the leg 54 of the H-tube 56, with a suitable neutral solution such as sodium sulphate in the other leg 55. In this case, the acidity at the cathode 2| may be reduced in accordance with the coulombs of electricity which produces the electrolysis. When an end-point is reached as indicated by a suitable indicator, the switch 51 will be opened and the coulombs determined from the meter 24a, or the acidity may be read directly therefrom.

As before, if the electrolytic action proceeds beyond the end-point the reversing switch 51 is operated to reverse the direction of current fiow and to reverse the chemical reactions until the acidity within the leg 55 has brought the solution to the desired end-point. Again the meter 24a measures the coulombs of electricity for the reverse operation. These are again subtracted from those which produced the original change in the compositional characteristic for the net determination of the coulombs in terms of the original alkalinity of the solution.

Further, in accordance with the invention, solutions which contain a known quantity of a reagent and standardized solutions may be produced by electrolysis. There is a definite, proportional relationship between the quantity (the coulombs) of electricity and the quantity of reagent electrolytically produced. Therefore, by utilizing any of the modifications described, a desired solution of known strength may be produced. Where the anodic and cathodic reactions are separated, as in Fig. 5, there is placed in one or the other of legs 54 and 55 of the H-tube 56 a solution, with an added agent (if necessary) 21 3 for electrolytic production of the reagentdesired itherein. Anexact quantity of reagentmaybe produced by applying a predetermined number .oflcoulombs. Without further measurements or other ;considerations, the solution, after said electrolysis, will have: the predetermined; quantity of ,reagent and :may :thereafter be utilized standardizing .a 1 solution. Of course, when the -.-reaction;at-the anode or .oathode doesnot adversely afiect the yieldof the desi red;reagent,the systemzofihig. .1 may be similarly ut l ed- .1;If ,thevolume of the solution which contains the ::.known. quantity of the electrolytically produced :rcagent ;is known. .then jits concentration .alsoiaknown. ilhis s lu onis st ndardolu- -tion,-;since:itsconcentration is known-and it may be :usedzto standardize another solution or ;to v:efiect a .volumetric :analysis.

:Specifically in ordertdn pare a-.known;solution. of iodine, 1a solutionnf p tass um d d -w ll Ice-introduced inthelegs Manda-55. Theconcentrationof the potassiumiiodid soluti n ue dnot be .known. :but potassium-iodidem st b pr sent in excess of the requirements "of the -electrolytic process. The cathodeil willcomprise a coating of,silverl(;h10ride on ;a:b ase 10f silver ,While the anode. 20 may b 10f an ;inert material such as platinum. Under electrolysis iodine will be zformed-za-t:the;anode,.:20. After the passageof .a

-l nown-:quan ity of current-for a known timefth amount of iodine produced will be ,;-l n cw n. :Hence, itzis, only necessaryito drain or pour from -the:1eg 55 :the, solution contained therein. for exampla-it will be assumed that a constantcur- :rent of ;0;0.5,00 ampere ,is caused to fiowior a period.-of:1'800,seconds. Theiamount of :lodineis ,givemby Equation :lrabove set-forth. Specifically,

-=volume of 'theabove solution was 0.04.50 liters,

ithenits normality isgiven as follows:

By way of a further example, in order to prepare a solution whichcontains a known amount of a ferrous salt, gferrous sulphate for example, the apparatus of Fig. 5 is filled with a solution which contains ferric sulphate in excess of the desirous amount of ferrous sulphate to be prepared. The switch 51 may be closed in either direction and the electrode which becomes the .cathode effects the conversion of ferric ions to fQI'lZQjllSgiOQS. ;It is necessary, however, to control .thepotentialat the cathode-so as-not to exceed t-hatuwliich-wouldpause the deposition of iron. a ib -acQm- 1 he b us n a cathode of demo a ea and b a ju t the setting of t e ass tanc 1 55. 1 t l m t th current. To be rusetu "thi solut on n ed not h v all cf the ".314 ferric sulphate converted to ferrous sulphataa-nd the electrolysis may be stopped at the desired point. The amount of ferrous sulphate in equivalents .whichwill be produced in the solution is given by Equation 1,,namely:

Suppose that the meter .2401. showed that386 coulombs hadpassed, thenthe ferrous sulphate in equivalents would be ;If 1the-volume of the ferricsulphate:solution .which-iwas'placed in the cathodic compartment of :the H -tube ;56:was measured, then thenormal- ,ity;-of;the;resultantferrous sulphatesolution may .be expressedby- Equation 3; namely Q T= A eascov Letthe volume of the solutionbe 0.0200 liter, he

The inertgas is released from the open endof the tube (if! and .bubbles upwardly through the solution andaround the detecting electrodes 26a and 21a. The flower gas induces circulation of the-fluid fromthe electrodes 20a and;2 la toelectrodes ZBaandJZ'Fa.

'Thereihas already-been described the measurement of the concentration ofhydrochloricjacid in a solution. Inaccordance with the invention; the

concentration of the chloride ions in a solution may also be determined. For example, there, may .be introduced into the vessel M of Fig. 6 aknown volume of a solution bearing the chloride ions, such, for ;eXample, as sodium chloride, and an addition-agent such as sodium nitrate. 'In this case the anode 20a will be silver and the cathode 21c, will be an inert material such as platinum .or'thelike. At the anode,the electrolytic action produces silver chloride. By utilizing a convenational reference electrode 26a, for example, of .calomel or the like, in conjunction with a measuring electrode 21a of silver, the end-point may-be determined. The afore-mentioned silver chloride istformed by the release of silver ions at the anode which combine .with the chloride ions. "When the ;chloride ions have been depleted,there appears in the solution an increase in the silver ions. As this excess appears, the detecting electrode 2'ia produces ,a characteristic potential which indicates itl earrival of the solution at its end-point.

;E0r example, suppose that the circuit of Fig. 5 is employed in measuring the chloride ion, or sodium chloride concentration, and that the cur- .rent is constant and numerically equal to 0.175 -am e :eand flowsfio a period of 2100 seconds.

Also let the volume of solution be 0.035 liter. Then Equation 4 is again used:

which is normality of the original solution with respect to sodium chloride.

Since in the foregoing example, Fig. 6, the anode a and the measuring electrode 21a are both of silver, only one of them need be provided in certain cases. Under certain conditions the single electrode, either 20a or 21a, will perform both functions described for electrodes 20a and 21a. In the three-electrode modification, the outer face of cylindrical electrode 21a (or 20a) will function as the anode in co-operation with the cathode 2Ia. The inside face of cylindrical electrode 21a (or 20a) will function as the detecting or measuring electrode in cooperation with the conventional reference electrode 26a. Hence the three electrodes 26a, 21a (or 20a), and He will perform in this instance all of the functions described for the two pairs of electrodes Net-21a and Mia-21a.

The invention is not limited to the determination of the compositional characteristic of solutions or of solids suspended in solutions. The invention also lends itself to continuous or intermittent gas analysis. More specifically, there is shown in Fig. 7 an electrolytic titrometer in which gas is fed to the system by way of the pipe (52, having a distributing head 63 at the lower portion thereof. Gas discharged from the distributing head flows upwardly around and between cylindrical electrotitration electrodes 64 and 55. The outer electrode 65 is provided with a series of openings 66 for exit of the gas. The gas, in the form of a series of bubbles, flows outwardly through the openings and up the inclined tubular connection 61, and is discharged from the system by way of pipe 68. The gas not only intimately diffuses through the electrolyte 69 but it also induces circulation of the electrolyte about a circuit including the titration electrodes 64 and 65, the inclined tubular connection, and the vertical tube H3. Within the tube 70 are mounted a reference electrode H and a detecting electrode 12. Though the titration electrodes and the reference and detecting electrodes are separated from each other, the foregoing circulation of the electrolyte minimizes time delay due to changing compositional characteristics of the gas undergoing analysis.

In accordance with the invention, there may be readily determined the quantity of gas, liquid, or solid present in a gas. This determination is highly sensitive and by means of the invention it is possible to detect impurities which may be present only in slight degree. If the amount of sulphur dioxide present in a gas is to be determined, an electrolyte is provided in the system of an alkaline character, with an added agent which supplies iodide ions. Specifically, the electrolyte may be sodium hydroxide in mixture with sodium sulphate, with an added agent of sodium iodide.

It will be assumed that a gas which is suspected of containing sulphur dioxide is inert with respect to the chamical reactions which take place in the electrotitration system. For example, let it be air contaminated with sulphur dioxide. Either a measured quantity of air is injected into the electrolyte or air is fed into the electrolyte at a uniform rate. If the concentration of sulphur dioxide in the air is very small, the preferred method of operation will be continuously to supply the air through the pipe 62, Fig. 7, so that it continuously bubbles through the electrolyte 69. Measurements in accordance with the invention are then conducted at desired time intervals, or the measurements are made continuously as in the example of the continuous analysis of the hydrochloric acid solution.

More specifically, the vessel 61 will be filled with the aforesaid electrolyte. The sulphur dioxide, in mixture with air, is introduced through the head 63. The sulphur dioxide unites with the sodium hydroxide to form sodium sulphite. The sodium sulphite is then determined in manner already described. It will not be necessary continuously to add sodium iodide because iodide is regenerated in the reaction. For continuous operation, it will be necessary to make up for the loss of the sodium hydroxide which reacts with the sulphur dioxide. There will also be a loss of water. Both can be added as needed to maintain the volume constant.

It is to be understood that other means of detecting the end-point may be utilized, such as the addition of a starch solution which, upon the attainment of the end-point, is distinctively colored by the iodine. In case the concentration of sulphur dioxide is substantial, the system may be operated in much the same way as has been set forth in connection with Figs. 1 and 2. The electrotitrating current will be applied continuously so as to yield the titrating reagent in quantity which will maintain the solution at its end-point. Knowing the rate of flow of the gas and the rate of flow of the necessary titrating current, the amount of sulphur dioxide present will be continuously indicated. It may be further mentioned that a batch-operation may also be conducted where a predetermined quantity of gas is injected into the electrolyte, followed by electrotitration for determination of the sulphur dioxide.

The determination of the concentration of sulphur dioxide in air may be accomplished by using the equations previously given. For a continuous analysis of sulphur dioxide in air, Equations 5 and 2 are combined, eliminating N:

E is the equivalent of sulphur dioxide in the volume V of air. I is the current employed, and R is the rate of flow of the air into the chamber of Fig. '7. The units of R must be in volume, as V (liters), and time, seconds.

Let a current of 0.052 ampere be required in the electrolytic titration and a flow of air into the cell be 0.020 liter per second. Then:

g 0.052 V 96,500X0.020

or, 2.'7 10 equivalent of sulphur dioxide per liter of air.

If hydrogen sulphide is to be quantitatively determined, for example the amount of hydrogen sulphide in air, the foregoing procedure is repeated. The electrolyte will again be an alkaline solution in which the hydrogen sulphide will be absorbed. For example, the electrolyte may be a solution of sodium acetate. The anode 65 will be selected of a metal, as of zinc, which forms afiecting the concentration of the constituent in said stream, continuously withdrawing from said stream a sample after addition of said reagent, passing said sample through a treating zone at a measured rate of flow, passing direct current at a measured rate through said sample in said treating zone electrolytically to change the concentration of said constituent in said zone, and then varying the rate of addition of said reagent in accordance with departure in concentration of said constituent in said sample from a predetermined value until said direct current brings said concentration of said sample to said predetermined value.

5. The method of continuously determining the concentration of a constituent in a solution which comprises producing a measured rate of flow (R) of said solution through a treating zone, passing direct current (I) at a measured rate through said solution while in said treating zone electrolytically to change the concentration of said constituent in said solution, and then varying at least one of said rates of flow to bring the concentration of said constituent to that predetermined value at which the normality (N) of the solution may be expressed as times a constant.

6. In a system for continuously determining the concentration of a constituent in a solution, the combination of a flow channel forming a treating zone for passage of said solution therethrough, means for delivering said solution to said zone, a flow controller for varying the rate of flow of said solution through said zone, means including a pair of electrodes disposed for flow of direct current therebetween electrolytically to vary the concentration of said constituent in said solution during its passage through said zone, means in circuit with said electrodes for measuring said direct current, and means for varying the rate of at least one of said flows to bring said concentration of said constituent to a predetermined value.

7. In a system for continuously determining the concentration of a constituent in a solution, the combination of a flow channel forming a treating zone for passage of said solution therethrough, a flow controller for predetermining the rate of flow of said solution through said zone, means including a pair of electrodes disposed for passage of direct current therebetween and through said solution electrolytically to vary the concentration of said constituent in said solution during its passage through said zone, means in circuit with said electrodes for measuring said direct current, and means for varying the rate of flow of said direct current between said electrodes until arrival of said concentration of said constituent at a predetermined end-point.

8. A system for continuously determining the concentration of a constituent in a solution comprising a container having an inlet and an outlet for passage of fluid therethrough, a flow controller for delivering said solution to said container for flow therethrough at a rate determined by said flow controller, a pair of current electrodes for passage of direct current through said solution electrolytically to vary the concentration of said constituent of said solution during passage of said solution through said container, means in circuit with said current elec- 20 trodes for measuring said current, and means for varying the rate of flow of said direct current between said current electrodes until arrival of said concentration of said constituent to a, predetermined end-point.

9. In quantitative analysis by titration involving the quantitative reaction in an electrolyte of a titrating agent with a constituent to be measured, the improvement which comprises regulating the amount of titrating agent present in the electrolyte by electrolysis in situ, measuring the current employed in the electrolysis to give a measure of the rate of consumption of titrating agent in the reaction, sensing the unreacted amount of one of the reactants, and varying the electrolytic current responsive to variations in said unreacted amount of said one of said reactants to maintain the concentration of said one of said reactants at a fixed level.

10. In a system for continuously determining the concentration or" a constituent in a fluid, the combination of a vessel containing electrolyte forming a treating zone for passage of said fluid therethrough, a flow controller for controlling the rate of flow of said fluid through said zone, means including a pair of electrodes disposed for flow of direct current therebetween electrolytically to vary the concentration of said constituent in said zone during its passage through said zone, means in circuit with said electrodes for measuring said direct current, and means for varying the rate of at least one of said flows to bring said concentration of said constituent to a predetermined value.

11. In quantitative analysis by titration involving the quantitative reaction in an electrolyte of a titrating agent with a constituent to be measured, the improvement which comprises regulating the amount of titrating agent present in the electrolyte by electrolysis in situ, measuring the current employed in the electrolysis to give a measure of the rate of consumption of titrating agent in the reaction, sensting the unreacted amount of one of the reactants potentiometrically, and varying the electrolytic current responsive to variations in the sensed potential to achieve a fixed concentration of said one of the reactants.

12. A system of quantitative analysis comprising a vessel for electrolyte containing a constituent to be measured, a pair of generating electrodes disposed within said vessel for flow of direct current betweeen them electrolytically to change the concentration of said constituent in said electrolyte, current measuring means, a current regulator having a sensing circuit and a direct current power circuit including said generating electrodes and said current measuring means, a pair of voltage-producing sensing electrodes neither of which carries said direct current disposed in said vessel, means including said sensing electrodes connected in said sensing circuit to produce therein a voltage of magnitude related to the concentration of said constituent in said electrolyte, and said regulator including means for establishing between said generating electrodes flow of direct current of magnitude dependent upon a difference between an endpoint potential and the voltage produced by said sensing electrodes and independent of the potential of either of said generating electrodes to bring the concentration of said constituent in said electrolyte to a predetermined value corresponding with that represented by said endpoint potential.

13. A system of quantitative analysis comprising a vessel for electrolyte containing a constituent to be measured, a pair of generating electrodes disposed within said vessel for flow of direct current between them electrolytically to change the concentration of said constituent in said electrolyte, current measuring means, a current regulator having a sensing circuit and a direct current power circuit including said generating electrodes and said current measuring means, a pair of voltage-producing sensing elec-- trodes neither of which carries said direct current disposed in said vessel, means including said sensing electrodes connected in said sensing circuit to produce therein a voltage of magnitude related to the concentration of said constituent in said electrolyte, said regulator including means for establishing between said generating electrodes flow of direct current of magnitude dependent upon a difference between an end point potential and the voltage produced by said sensing electrodes and independent of the potential of either of said generating electrodes to bring the concentration of said constituent in said electrolyte to a predetermined value corresponding with that represented by said endpoint potential, and electrical isolating means interposed between said power circuit and said sensing circuit in all portions thereof external to the electrolyte in said vessel.

EDGAR L. ECKFELDT.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS OTHER REFERENCES Outlines of Theoretical Chemistry, 6th edition (1937), by Getman et al., page 477.

Transactions of the Electrochemical Society," vol. 76 (1939), pages 303 thru 324.

Osterreichische Chemiker Zeitung" pages 217 thru 223.

Hackhs Chemical Dictionary, 2nd edition (1937), page 948.

Transactions of The Faraday Society, vol. 38 (1942), pages 27 thru 33. 

